In 2000 exit polling asked:
If these were the only two presidential candidates, who would you vote for? (Bush got 49%, Gore 48%, and not voting 2%) Of those who picked Gore, 2% were Nader voters, of those who picked Bush, 1% were Nader voters, and about 1/3 of the not voting group were Nader voters.
So it looks like Gore would have gotten a net excess of about 20% of Nader votes, enough to clearly win FL (of course), but not NH or any other state.
However, if more votes from Nader would go to Gore than Bush, why did Bush get more votes in the exit poll than Gore? Because, strangely (amazingly?), if Nader was not running, 2% of the Gore voters said they would have voted for Bush!
See MSNBC for additional polling results:
http://www.msnbc.com/m/d2k/g/polls.asp?office=P&state=N1